Radio receiver circuits



y 1941- v R. A. WEAGANT 2243;140

RADIO RECEIVER cIRcuI'Ts Filed Oct. 14, 1938 2 Sheets-Sheet 1 ZE DETECTOR .4 VC RECTIFIER -INVEN TOR. ROY A. WEA GAN T A TTORNEY.

May 27, 1941- R. A. WEAGANT RADIO RECEIVER CIRCUITS Filed Oct. 14, 1938 2 Sheets-Sheet 2 T0 GRID 31 0F 77/5530 2 T0 TUBE 30 70 RE AND I F GRIDS INVEN TOR. R0 jig E7161? NT BY A TTORNE Y.

Patented May 27, 1941 UNETED STTES ATENT OFFHCE RADIO RECEIVER CIRCUITS Roy A. Weagant, Donglaston, Long Island, N. Y., assignor to Radio Corporation of America, a

corporation of Delaware 11 Claims.

My present invention relates to radio receiver circuits, and more particularly to receivers of high selectivity and sensitivity.

One of the main objects of my present invention may be stated to reside in the provision of a radio receiving system which will have a relatively hi'gh selectivity against strong adjacent channel interference, and simultaneously be highly sensitive for the reception of weak, or distant, signals without impairing the fidelity of reproduction of such latter signals.

Another important object of my present invention is to provide a radio receiving system which is capable of substantially reducing static, and other spurious electrical disturbances, and is, furthermore, adapted to minimize the elfects arising during selective fading.

Another important object of the present invention is to provide a receiving system of the type employing balanced detectors having oscillations injected into a common input circuit in such phase relation to the modulated signal carrier input so as to produce a high discrimination against undesired electrical impulses; the injected oscillations being derived from or controlled by the received signal carrier energy and remaining in frequency synchronism with the carrier frequency throughout a wide range of signal carrier amplitude variation.

Another object of my invention may be stated to reside in the provision of a superheterodyne receiver of the type employing a normal degree of selectivity preceding the audio demodulator network, and which is yet so constructed in the latter network as greatly to improve the overall selectivity of the receiver against undesired electrical impulses; and the audio demodulator network comprisin a pair of detectors having input and output circuits arranged in balanced relation for received modulated signal carrier energy, and a common input circuit upon which is impressed oscillations of a frequency equal to the carrier frequency; the injected oscillations being derived from the received carrier energy and being subjected to amplitude control in response to carrier amplitude variations.

Still other objects of my invention may be stated to reside in the provision of a highly improved superheterodyne receiver which not only embodies the various characteristics enumerated above, but which is moreover economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 shows a receiving system embodying one form of the invention,

Fig. 2 illustrates a modified form of audio demodulator network and AVG circuit.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 a receiving system of the superheterodyne type, and such system, except for the novel features to be described in detail at a later point, is conventional in nature. The receiver may, for example, comprise a signal collector device such as an antenna I; however, it is to be understood that there may be used in place thereof a radio frequency distribution line, a loop antenna, or the type of antenna commonly used on mobile structures, such as automobiles. The collected intelligence-modulated carrier energy may be amplified in one or more stages of tunable radio frequency amplification 2, and the amplified carrier energy may then be impressed on a first detector 3 which has a tunable input circuit. A tunable local oscillator network 4 is usually employed to provide the local oscillations of a frequency differing from the signal carrier frequency by the operating intermediate frequency (I. F.).

The I. F; energy, produced in the output circuit of first detector 3, may then be amplified in one or more stages of I. F. amplification 4. It is to be understood that the modulated carrier energy may be chosen from the broadcast range of 500 to 1500 kc., and that the I. F. may be chosen from a range of to 450 kc. Of course, the receiving system is also capable of use in the short wave ranges, and as a matter of fact finds particular value in that frequency range employed for commercial radio reception. The numeral 5 designates the tuned output circuit of the second I. F. amplifier, and it will be understood that the condenser 6 thereof resonates the coil 7 to the operating I. F. value. The second detector network, or audio demodulator, comprises a pair of electron discharge tubes 8 and 9, and inthe present case they are shown as being of the triode type. of course, if desired, a twin-triode type of tube can be used in place of separate tubes.

The grid IU of tube 8 is connected to its cathode through a series path comprising condenser II, coil I2, lead I3, condenser I4 and coil I5; the cathode end of coil I5 is established at ground potential. The grid I8 of tube 9 is connected to its cathode through a series path including condenser I'I, coil I8, lead I3, condenser I4 and coil I5. The adjustable condenser I9 is connected in shunt with coil I2, while the adjustable condenser is connected in shunt with coil I8. Both condensers I9 and 29 are in series relation, and the junction thereof is connected to the junction of coils I2 and I8 and to one terminal of condenser I4. Grid leak resistor 21 is connected between the opposite terminal of condenser I4 and the grid side of condenser II, while grid leak resistor 22 is connected between the grid side of condenser I1 and condenser I4.

Coil 1 functions as a primary winding, and is magnetically coupled to each of coils I2 and I8. There is no magnetic coupling between coils I2 and I8. Each of condensers I9 and 20 is adjusted, by a factory adjustment, so as to fixedly resonate its associated coil to its operating I. F. value. The coil I5 is shunted by a condenser 23 which functions to resonate coil I5 to the operating I. F., and it will be noted that the cathodes of tube 8 and 9 are connected to the grounded end of coil I5. The plate of tube 8 is connected to one end of the primary winding 24 of the electrostatically screened audio transformer 25, .while the plate of tube 9 is connected to the opposite end of winding 24. Each of the plates of tubes 8 and 9 is energized by connection to a source of proper positive potential, and which source is omitted to preserve simplicity of disclosure.

The secondary winding 28 of the audio transformer may be connected to one or more stages of audio amplification followed by any desired type of sound reproducer. If desired, a variable resistor 29 may be shunted across the output of the secondary winding 28, in that case the variable resistor will function as a manually adjustable audio volume control. It will be appreciated from the above description that tubes 8 and 9 have associated with them circuits which render them capable of operating as a pair of grid leak detectors whose signal input and output circuits are in balanced, or push-pull, relation. It will be appreciated that in such a balanced detector network having a differential output circuit, the radio frequency input to both detectors is equal so that all normal detection of desired signals, interference and static disturbances is suppressed. It is only by the injection of the properly phased oscillation, of a frequency equal to the signal carrier frequency, that an audio output is secured in the receiver; and, moreover, the injection of the local oscillations must be arranged so that the frequency thereof stays constantly in synchronism with the received carrier frequency.

This is accomplished in a highly simplified and efficient manner by providing a tube 30 whose grid 3| is connected through condenser 32 and lead 33 to the high potential end of the primary winding I. The cathode of tube 30 which may be a pentode of the 6K7 type, may be connected to ground through the usual self-biasing resistorcondenser network 33; the plate 34 of tube 30 is connected, through a path including lead 35 and coil 36, to a source of proper positive potential. The coil 35 is shunted by a condenser 31 which resonates-the former to the operating 1. F. Coils I5 and 3B are magnetically coupled,

and there is thus provided a tuned network, resonated to the operating I. E, which functions as the injection point for the oscillations of carrier frequency.

The tube 39 is provided with reactive coupling between its plate 34 and grid 3I so that it may function regeneratively to amplify the I. F. carrier. For this purpose reliance may be placed on the internal grid to plate capacity of tube 30; however, a special adjustable condenser 40 may be utilized between plate 34 and grid 3I for this purpose. Condenser 40 will be adjusted so that tube 38 functions to produce oscillations of the I. 13. value; it will be appreciated that these oscillations are derived from the incoming modulated I. F. carrier energy. The modulationside bands of the I. F. carrier energy will be effectively removed by the action of the tube 30, and there will be thus injected into the common input circuit I5-23 oscillations of I. F. value and free of modulation frequencies. Moreover, it is found that the injected oscillations will remain in frequency synchronism with the modulated I. F. carrier frequency throughout a wide range of I. F. carrier amplitude variation. Even for exceedingly weak signal reception such synchronism will be maintained.

An automatic volume control circuit (AVC) may be provided for the receiver in any well known manner. One mode of securing AVC bias, for example, involves utilizing the rectifier diode tube 50; the tube being, for example, of the type employing a pair of anodes 5I and 52 associated with the common cathode 53. Anode 5| is connected to the high potential end of coil I2, While anode 52 is connected to a similar end of coil I8. The cathode 53 is at ground potential, and the AVG load resistor 54 is connected between ground and the junction of coils I2 and I8. The ungrounded end of load resistor 54 may be connected, in a manner well known to those skilled in the art, to the signal grids of the radio and intermediate frequency amplifiers, while the load resistor 54 has an I. F. bypass condenser 55 in shunt therewith.

An improved AVC action is secured by controlling the gain of tube 30 in response to the variation in direct current potential developed across resistor 54. This is accomplished by connecting grid 3I to a desired point, of negative potential with respect to ground, on resistor 54 through a path which includes the filter resistor 60 and lead 6|. It will, therefore, be seen that for weak signal reception the oscillations injected into network I5-23 will be of maximum amplitude, whereas for signal carrier amplitudes of increasing intensity the AVG circuit will function to reduce the gain of tube 30 with the result that the oscillations injected into circuit I5-23 will be reduced in amplitude. An additional manner of regarding the action of the AVG circuit on the regeneratively connected tube 30 is to consider that the action of the AVG is to convert the regenerative network from a very low resistance circuit for weak signals into a normal resistance circuit for strong signal reception. Such an action results in an augmented AVC which is of particular value in the case of receiving systems employing a minimum of controlled amplifiers preceding the second detector.

When employing an AVC circuit as shown in Fig. 1 it is. necessary to utilize the series condenser I4, since this permits the connection of the load resistor 54 to the junction of coils I2 and I8. It will, also, be observed that the connection through condenser 32 of the grid 3! of the regenerative tube 39 to the primary 1. F. circuit 5 enables the user to secure proper phasing relations without the need for additional phasing networks. Again, condensers l9 and permit accurate balancing of the grid circuits of each of the detectors 3 and 9. In order to get the maximum effect from the regenerative tube 30, the adjustments thereof should be such that when the grid bias of the tube is correct for maximum amplification the tube is just below the oscillation point. In this condition the condenser M should have maximum value.

There will now be given an explanation of the theoretical aspects underlying the observed functioning of the aforedisclosed radio receiving system. However, it is to be clearly understood that the theoretical explanation of the circuit functioning in no way limits the invention, since the advantages and characteristics outlined heretofore, and to be explained hereinafter, have been observed in actual usage of a receiving system embodying the circuit outlined.

It can be demonstrated that with symmetrically arranged detectors of the type shown in Fig. 1, normal detection of desired signals, interference and static is suppressed, because the radio frequency input voltage impressed on each detector grid is of the same magnitude. The radio frequency voltage injected into the circuit l5-23 is of such phase as to increase the radio frequency voltage on one detector grid, while opposing in phase the radio frequency volttage on the other grid thereby causing an unbalanced condition in the detector output circuit. The output of the detector network consists solely of beat frequencies formed between the injected carrier and the various signals. Most of the beats are super-audible, except for the desired signal, and hence cause no interference. It can be stated that the detected output is proportional to the product of the demodulated carrier resonance curve and the resonance curve of the signal input circuit. Hence, an extremely high selectivity is secured with this type of circuit. When it is realized that beats between side frequencies in normal detection result in audio frequencies existing in detector circuits which are not in the original modulation, then it is realized why the present balanced detector arrangement neutralizes these spurious frequencies and improves the quality of the audio output.

When the radio frequency input circuits of the two detectors are properly balanced, there will be no audio output when either of the two sources of input voltage act alone. If, however, signal input and oscillations of carrier frequency are injected into the detector network, and are in the same phase on one of the detector grids, an audio output will be secured. It is important that the ratio of the oscillations, or demodulated carrier, injected at l5-23 to the signal input be made as large as possible in order to get high selectivity, great amplification and high fidelity. When this relation is secured, then the audio output will be proportional to the prodnot of the modulated and unmodulated inputs to the detector. Further, the audio output will contain only those frequencies which are the result of the interaction of the carrier and side bands; beats between the modulation frequencies themselves do not appear in the etector output circuit. Static disturbances are materially reduced by virtue of the phase detection arrangement, the desired signal being of a given phase while static disturbances are of many phases. A very high order of selectivity results due to the fact that because the carrier from an interfering signal on an adjacent channel is kept out, there is no interference which is intelligible. Again, a marked improvement is found to take place when selective fading is present; the injection of the demodulated carrier increasing the amplitude of the signal carrier with respect to its side bands.

In Fig. 2 there has been shown a modified form of balanced detector network which may be employed where it is not desired to utilize a pair of independent secondary windings in the detector input circuit. A modified form, of AVG circuit is also illustrated. Here the oathodes of the two tubes are grounded, and con nected in common by an adjustable tap B0 to a common grid leak resistor 6| which is connected between the grids of the two tubes. mary winding l of theI. F. transformer is coupled to a coil 1' across which is connected the condenser 62 functioning to resonate the common input circuit of the detector network to the operating I. F. value. The midpoint 63 of secondary winding 1' is connected to the grounded cathodes of tubes 8 and 9 through the network lii-ZS, and the demodulated carrier energy is injected into network I5-23 by the I. F. tuned network Sit-31. A potentiometer of approximately 250,000 ohms may be employed to provide elements ISL-50. The operation of the detector network shown in Fig. 2 is similar to that described in connection with Fig. 1. It is to be understood that the network 30-37 will be connected to the plate circuit of the I. F. regenerative tube 30.

It is not necessary to employ the separate AVC diode 50 shown in Fig. 1. Since the grid of each detector tube cooperates with its cathode to provide a diode rectifier, the grid leaks of the balanced detectors can be used to supply the AVG bias. This arrangement will be of value in compact sets, as those of the midget type, and one form of the arrangement is shown in Fig. 2. Here the resistor 10 is shunted across the grid leak BI, and the AVG leads H and 12 are connected to the midpoint thereof. The resistor and grounded condenser function as a filter network to prevent pulsating voltage from reaching the grids of the controlled tubes.

While I have indicated and described several arrangements for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made. without departing from the scope of my invention as set forth in the appended claims;

What I claim is:

1. In a system for receiving modulated signal carrier energy and which includes a pair of detectors in relatively opposite phase relation whereby products of detection arising solely out of the signal voltages are balanced out of the detector output circuit, the method comprising impressing equal magnitudes of said energy upon said detectors, deriving from the received modulated carrier energy oscillations of a frequency equal to the carrier frequency, impressing the derived oscillations upon said detectors in such phase relation that the oscillations are in phase with the signal voltage in one of the detectors, deriving a control'voltage from the modulated The prisignal carrier energy, and controlling the amplitude of said oscillations by controlling the efficiency of said oscillation-derivation with said control voltage.

2. In a receiving system of the type including a plurality of selective amplifiers, a demodulation network comprising a pair of detectors having output circuits arranged in opposition, said detectors having input circuits coupled to the last of said ampifiers, means consisting of a regenerative amplifier coupled to said last amplifier for deriving amplified oscillations therefrom which are of the signal carrier frequency, means comprising a highly selective network tuned to said carrier frequency for impressing said derived oscillations upon said detector input circuits so that the phase of the impressed oscillations is the same as the phase of the signals impressed on only one of said detectors.

3. In a receiving system of the type including a plurality of selective amplifiers, a demodulation network comprising a pair of detectors having output circuits arranged in opposition, said detectors having input circuits coupled to the last of said amplifiers, means coupled to said last ampifier for deriving oscillations therefrom which are of the signal carrier frequency, means for impressing said derived oscillations upon said detector input circuits so that the phase of the impressed oscillations is the same as the phase of the signals impressed on only one of said detectors, said deriving means consisting solely of a regeneratively coupled tube having a highly selective output circuit coupled to said detector input circuits.

4. In a receiving system of the type including a plurality of selective amplifiers, a demodulation network comprising a pair of detectors having output circuits arranged in opposition, said detectors having input circuits coupled to the last of said amplifiers, a regenerative amplifier coupled to said last ampifier for deriving amplified oscillations therefrom which are of the signal carrier frequency, means for impressing said derived oscillations upon said detector input circuits so that the phase of the impressed oscillations is the same as the phase of the signals impressed on only one of said detectors, andv means responsive to variation in signal carrier amplitude for controlling the gain of said re generative amplifier thereby to adjust the amplitude of said derived oscillations.

5. In a receiving system of the type including a plurality of selective amplifiers, a demodulation network comprising a pair of detectors having output circuits arranged in opposition, said detectors having input circuits coupled to the last' of said amplifiers, a regenerative amplifier coupled to said last amplifier for deriving amplified oscillations therefrom which are of the signal carrier frequency, means for impressing said derived oscillations upon said detector input circuits so that the phase of the impressed oscillations is the same as the phase of the signals impressed on only one of said detectors, and means responsive to variation in signal carrier amplitude for controlling the gain of said regenerative amplifier thereby to adjust the amplitude of said derived oscillations, said last named means comprising a rectifier coupled to the last of said amplifiers and producing a control voltage, and means for impressing said control voltage upon at least one of said selective amplifiers to control the gain thereof.

6. In a receiving system of the type includsive to variation in signal carrier amplitude for controlling the amplitude of said derived oscillations, said deriving means comprising a tube having its input and output circuits regeneratively coupled, and including a connection between its input electrodes and the last of said amplifiers, and a highly selective network tuned to said signal carrier frequency coupling the output electrode of said last tube and said detector input circuits and said amplitude control means having a gain control connection to said regeneratively coupled tube.

7. In a receiving system of the type including a selective network adapted to pass modulated signal carrier energy, a detection network comprising a pair of carrier energy rectifiers provided with a common input circuit coupled in push-pull relation to said selective network, said rectifiers having a common output circuit adapted to develop modulation voltage, a regeneratively coupled tube having input electrodes coupled to a. point in the system prior to said rectifiers for deriving amplified oscillations of carrier frequency from modulated carrier energy, means impressing said derived oscillations upon said common input circuit with a high ratio of derived oscillation magnitude to modulated carrier voltage magnitude at said common input circuit, and means, responsive to variation in modulated carrier amplitude, for controlling the regenerative tube gain thereby to adjust said derived oscillation magnitude.

8. In a receiving system of the type including a selective network adapted to pass modulated signal carrier energy, a detection network comprising a pair of carrier energy rectifiers provided with a common input circuit coupled in push-pull relation to said selective network, said rectifiers having a common output circuit adapted to develop modulation voltage, a regeneratively coupled tube having input electrodes coupled to a point in the system prior to said rectifiers for deriving amplified oscillations of carrier frequency from modulated carrier energy, means impressing said derived oscillations upon said common input circuit with a high ratio of derived oscillation magnitude to modulated carrier voltage magnitude at said common input circuit, said ratio being sufiiciently high to cause said modulation voltage to be proportional to the product of said modulated carrier voltage and said derived oscillation voltage.

9. In a receiving system of the type including a selective network adapted to pass modulated signal carrier energy, a detection network comprising a pair of carrier energy rectifiers provided with a common input circuit coupled in push-pull relation to said selective network, said rectifiers having a common output circuit adapted to develop modulation voltage, a regeneratively coupled tube having input electrodes coupled to a point in the system prior to said rectifiers for deriving amplified oscillations of carrier frequency from modulated carrier energy, means impressing said derived oscillations upon said common input circuit with a high ratio of derived oscillation magnitude to modulated carrier voltage magnitude at said common input circuit, and means, responsive to variation in modulated carrier amplitude, for controlling the regenerative tube gain thereby to adjust said derived oscillation magnitude, said rectifiers having a :common load impedance in circuit therewith in said common input circuit thereby to develop a uni-directional voltage from modulated carrier energy, and said control means utilizing said uni-directional voltage.

10. In a receiving system of the type including a selective network adapted to pass modulated signal carrier energy, a detection network comprising a pair of carrier energy rectifiers provided with a common input circuit coupled in push-pull relation to said selective network, said rectifiers having a common output circuit adapted to develop modulation voltage, a regeneratively coupled tube having input electrodes coupled to a point in the system prior to said rectifiers for deriving amplified oscillations of carrier frequency from modulated carrier energy, means impressing said derived oscillations upon said common input :circuit with a high ratio of derived oscillation magnitude to modulated carrier voltage magnitude at said common input circuit, means, responsive to variation in modulated carrier amplitude, for controlling the regenerative tube gain thereby to adjust said derived oscillation magnitude, and a highly selective circuit, tuned to said carrier frequency, coupling said regenerative tube output electrodes to said common input circuit.

11. In a receiving system of the type including a selective network adapted to pass modulated signal carrier energy, a detection network comprising a pair of carrier energy rectifiers provided with a common input circuit coupled in push-pull relation to said selective network, said rectifiers having a common output circuit adapted to develop modulation voltage, a regeneratively coupled tube having input electrodes coupled to a point in the system prior to said rectifiers for deriving amplified oscillations of carrier frequency from modulated carrier energy, means impressing said derived oscillations upon said common input circuit with a high ratio of derived oscillation magnitude to modulated carrier voltage magnitude at said common input circuit, means, responsive to variation in modulated carrier amplitude, for controlling the regenerative tube gain thereby to adjust said derived oscillation magnitude, and said last mentioned control means comprising a carrier energy rectifier network coupled to said selective network and developing a direct current voltage, and means for applying the latter voltage to said regenerative tube as a gain-control bias.

ROY A. WEAGANT. 

